Circular frames of this type, in which yarns are selected at the yarn guides, are known. The space occupied by these yarn selecting devices at the bed limits the number of knitting systems and renders maintenance and regulating operations more difficult. Furthermore, as all the preselected yarns enter the selecting device, these yarns can only be conveyed upstream of the yarn guide by means of a disconnectable drive member associated with each of these yarns.
The problem is just as complex for rectilinear frames, as each preselected yarn must be associated with a respective yarn guide, all the yarn guides being stored at the ends of the bed and only the yarn guides of the knitted yarns being associated with the respective knitting systems. According to the number of knitting systems and the number of colors, and according to whether knitting is carried out in single or double panels, the number of yarn guides may be quite high and therefore present serious problems as regards construction and operation. In addition, it is practically impossible to convey these yarns between the selecting device and the yarn guide.
Solutions consisting in joining a yarn being knitted and a yarn selected for knitting have already been proposed.
The German Pat. No. 47290 has proposed the formation of an open loop on the yarn being knitted, then the formation of a yarn reserve before leading this yarn to the needles. When the new yarn is joined the yarn being knitted is stopped at the open loop, a loop is formed about this open loop with the yarn selected for knitting by passing this selected yarn into and then out of this open loop, after which the two loops are tightened to form the knot. During all these steps the knitting frame is supplied with the previously formed yarn reserve. It is therefore only necessary for the reserve to have a yarn length which is sufficient to provide the yarn length corresponding to the duration necessary for joining the two yarns. It is hardly possible to carry out an operation of this type in less than one second. If the yarn is supplied at a speed of 5 m/s, the reserve must store a length which is proportional to the duration necessary to effect a join, which generally presents problems as regards required space, which can only be reduced at the cost of a decrease in the yarn supply speed.
A proposal has been made for partially solving this problem of space by simplifying the knot necessary for joining the two yarns. For this purpose the German Offenlegungsschrift No. 3 015 191 relates to a mechanism in which the yarn to be knitted passes over the circumference of a drum and is moved along by a belt bearing against a portion of this drum. Positioning fingers serve to place the yarn being knitted and the yarn selected for knitting in the path of a groove which is rigid with the edge of the drum and adapted to release the yarn being knitted from the circumference of the drum and engage thereon the yarn selected for knitting. During this rotation of the drum, the ends of the two yarns retained inside the drum are knotted by a knotter of the conventional type, whereupon this knot is separated from these ends, so that the yarn selected for knitting is moved towards the knitting frame after the yarn being knitted.
In spite of the simplicity of the mechanism which uses a simple knotter, the diameter of the drum is determined by the yarn supply speed and by the duration necessary for the yarn changing operation, and the knotting operation, which generally lasts from 0.1 to 0.2 s, takes place over an angle of approximately 90.degree., so that the minimum duration of rotation of the drum is from 0.4 to 0.8 s. If a yarn is supplied at 4 m/s, a drum having a circumference of at least 1.6 m, i.e. a diameter of at least 50 cm, will be necessary. A diameter of this size is hardly acceptable in practice, so the supply speed of the knitting frame has to be greatly reduced.